The dynamics of the gel to fluid phase transformation in 100 nm large unilamellar vesicles (LUV) of 1,2-dipalmitoyl(d62)-sn-glycero-3-phosphocholine (d62-DPPC), has been studied by laser-induced temperature-jump initiation coupled with time-resolved infrared spectroscopy and by MD simulations. The infrared transients that characterize the temperature dependent phase transformation are complex, extending from the nanosecond to the millisecond time scales. An initial fast (submicrosecond) component can be modeled by partial melting of the gel domains, initiated at pre-existing defects at the edges of the faceted structure of the gel phase. Molecular dynamics simulations support the model of fast melting from edge defects. The extent of melting during the fast phase is limited by the area expansion on melting, which leads to a surface pressure that raises the effective melting temperature. Subsequent melting is observed to follow highly stretched exponential kinetics, consistent with collective relaxation of the surface pressure through a hierarchy of surface undulations with different relaxation times. The slowest step is water diffusion through the bilayer to allow the vesicle volume to grow along with its expanded surface area. The results demonstrate that the dominant relaxation in the gel to fluid phase transformation in response to a large T-jump perturbation (compared to the transition width) is fast (submicrosecond), which has important practical and fundamental consequences.
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